1. Field of the Invention
This invention relates to a means for light beam scanning in optical information readers, such as bar code readers. More particularly, the present invention relates to two-dimensional optical scanners, which enable multi-directional reading of bar code symbols, reading of multi-stage bar codes, and reading of two-dimensional bar codes.
2. Related Art
Recently, bar code readers have been widely used in various fields, e.g. POS (Point of Sale, i.e. check out counters) in supermarkets and product distribution. Various sizes of bar code labels, from small to large, appear on products. Bar code readers are classified according to the reading method used to gather information, such as the light-pen type, CCD image sensor type, and laser scanning type.
Among them, the bar code readers of the laser scanning type are widely used because they have the ability to read bar codes at a distance from the bar code label (remote reading) and can read the myriad of label sizes that are commonly found, by adjusting the distance between the label and the reader. The bar code readers of the laser scanning type can be classified according to operating style into the fixed type and the hand-held type and according to the scanning method into the one-dimensional scanning type and the two-dimensional scanning type. Moreover, the two-dimensional scanning readers are classified into the multi-directional scanning type (hereinafter referred to as the multi-scanning type) and the raster scanning type.
At first, the one-dimensional scanning type reader scans a bar code in one direction to read the bar code. Thus, it is necessary to align the reader with the bar code label or to set the bar code label so that it is disposed in the scanning direction of the laser beam. Since this type has a relatively simple scanning mechanism and is helpful in reducing the size of the reader, it is mainly used with hand-held readers.
On the other hand, the multi-scanning reader among the two-dimensional scanning type readers has the following advantages. The multi-scanning reader can read bar codes on labels applied in various directions because it emits a laser beam in various directions and, therefore, it is possible to read bar codes without aligning the bar code reader or bar code label to the scanning direction.
The raster-type scanning reader among the two-dimensional scanning readers can grasp two-dimensionally the data shown on the label, so it has become an essential technique for reading of two-dimensional bar codes, and has come into wide use in recent years. However, since this two-dimensional scanning mechanism is relatively complicated and requires a large size, it is difficult to be introduced in hand-held readers. Therefore, it has found few applications.
Recently, a new two-dimensional mechanism aiming at simplifying and downsizing the reader, e.g. a method of oscillating a mirror through relative movement of four resonating piezoelectric bimorphs was disclosed in Japanese Patent Application Laid-Open No. 4-505969, (equivalent to PCT international publication WO 90/01715) has been proposed. However, in the method disclosed in this document, one mirror is connected with four bimorphs by using rubber bearings. Thus, the obtained mechanism has a fragile structure. Setting of a large scanning angle may result in brittle fracture of piezoelectric bimorphs because the resonance of the piezoelectric bimorphs is utilized. Moreover, the resonant frequency of the piezoelectric bimorphs is considerably high, approximately 2 kHz, and scanning with a light beam at this speed requires a high-speed signal processing circuit for processing received light signals. The current technical level makes it difficult to incorporate the high-speed signal processing circuit into hand-held bar code readers.
Another method of obtaining a two-dimensional scanning pattern is disclosed in Japanese Patent Application Laid-Open No.4-140706. In this document, a method has been proposed in which elastic flexible parts which can vibrate freely in two directions, i.e. bending and torsional directions, are resonated by one laminated piezoelectric element. The light beam is emitted onto the oscillating surface to obtain a two-dimensional scanning pattern. Since this method resonates the elastic flexible parts by a driving means and provides the scanning section which can be rotated at least in one direction by elastic vibration of the elastic flexible parts, the amplitude can be increased by adjusting the resonance of the elastic flexible parts, and any desired resonant frequency can be obtained quite easily by properly designing the oscillator. Thus, this method solves the above-mentioned problems.
Nevertheless, the method disclosed in the above-mentioned application has a disadvantage, that is the scanning pattern may vary when acceleration is added as a disturbance because an oscillating force is generated by inertial forces obtained by applying oscillation at a resonant frequency to the unbalanced structure. Therefore, this method is considered to be rather difficult to apply with such equipment as hand-held bar code readers.